1. Field of the Invention
This invention relates to the extraction and recovery of metal ions from an aqueous solution and in particular to the extraction of metal ions from an aqueous solution at either ambient or elevated temperatures using a composition containing immobilized microorganisms, and to the recovery of the bound metal ions from the composition.
2. Description of the Prior Art
The binding of metal ions to microorganisms and the application of this phenomenon to extract metals from an aqueous solution are rapidly growing areas of interest. Two distinct approaches to extraction of metals from an aqueous solution have been used. The first approach uses living organisms and the second approach uses a nonviable biomass Metal ion binding to living cells can occur either through surface adsorption or active intracellular accumulation. Metal ion binding to nonviable cells, however, is presumed to occur exclusively through surface adsorption.
Growing algae in ponds or lagoons for wastewater treatment is an example of using living organisms to extract metal ions from an aqueous solution. The basic approach has been to flow polluted waters through a lagoon in which an algal bloom is present. Because the algae adsorb heavy metal ions, the effluent waters from such a system have lowered heavy metal ion concentrations.
There are significant practical limitations to methods which employ living algal systems. The most significant limitation is that algal growth is inhibited when the concentrations of metal ions in the water are too high or when significant amounts of metal ions are sorbed by the algae.
Methods for water-treatment that employ nonviable cells, a biomass, are not complicated by the problem of attempting to maintain algal growth under adverse circumstances. In fact, heat-killed cells display a binding capacity for uranium(VI) three times greater than that measured for living cells. See for example, T. Horikoshi, A Nakajima and T. Sakaguchi, Agric. Biol. Chem., 43, p. 617, 1979. The biomass is treated merely as another reagent, a surrogate ion-exchange resin. The binding, or biosorption, of metal ions by the biomass results from coordination of the ions to various functional groups in or on the cell. These chelating groups--contributed by carbohydrates, lipids and proteins--include carboxyl, carbonyl, amide, hydroxyl, phenolic, imidazole, phosphate, amino, thiol, and thioether moieties.
Nonliving Rhizopus arrhizus, a common fungus, has been used for binding of U(VI) and Th(IV) in an aqueous solution. See for example, M. Tsezos and B. Volesky,. Bio-Technol. Bio-Eng., 23, p. 583 (1981). Frozen or freeze-dried preparations of Ulothrix, Chlamydomonas and Chlorella vulgaris have been used to remove Cu.sup.2+, Pb.sup.2+ and Zn.sup.2+ from an aqueous solution. The binding of these metal ions was greater at pH 7 than at pH 3. Moreover, NaCl and Mg(NO.sub.3).sub.2 inhibited the binding of zinc, suggesting that selective adsorption of Pb.sup.2+ or Cu.sup.2+ was possible. See for example, J. Ferguson and B. Bubela, Chem. Geol. 13, p. 163 (1974).
The algal species Rhodymenia palmata and Phorphyra yezoensis, both red marine algae; Laminaria japonica, Eisenia bicyclis, and Macrocystis pyrifera, all brown marine algae; Cyanidium caldarium, an acidophilic alga whose classification may be either green or red; Spirulina platensis, a freshwater blue-green alga; and Chlorella pyrenoidosa and Chlorella vulgaris, both freshwater green algae, have been found to adsorb tetrachloroaurate(III). However, the kinetics, pH dependencies and binding capacities differ amongst the algal species. Further, temperature was found to strongly affect gold binding to Spirulina platensis and to Chlorella pyrenoidosa. Increased gold(III) binding and reduction of gold(III) to gold(0) occurred as the temperature was increased from 0.degree. C. to 60.degree. C. This temperature dependence has been demonstrated for both free algal cells in batch experiments and immobilized algae See for example, D. Darnall, B. Greene and J Gardea-Torresdey, "Gold Binding to Algae" in Biohydrometallurgy, Kelley, D.P. and Norris, P.R., Editors, Science and Technology Letters, Kew Surrey, England, pp. 487-498 (1988).
Chlorella vulgaris has accumulated both gold(I) and gold(III) from aqueous solutions with high affinity. The degree of gold adsorption strongly depends on competing ligands present in the solution. Tetrachloroaurate(III) and gold(I) sodium thiomalate are rapidly adsorbed by the algal cells over a wide pH range, whereas dicyanoaurate(I) is bound more slowly and in a highly pH-dependent manner, with maximum binding observed near pH 3.0. Under certain conditions, the level of gold accumulation by Chlorella vulgaris approaches 10% of the organism's dry weight. Experiments suggest that the alga rapidly reduces gold(III) to gold(I) and that the algal-bound gold is slowly reduced to gold(0).
In addition to the specific algae cited, the bluegreen, green, brown, euglenoids, stonewarts, golden, dinoflagellates and red algae and other microorganisms such as bacteria, fungi, yeast or other plant materials have metal ion-binding capability.
The cell wall composition of members of the various algal groups is known to be diverse. The surface of the alga Chlorella vulgaris, a freshwater green alga, is literally a mosaic of metal ion binding sites--sites which differ in affinity and specificity. Both anions and cations can be bound. There are sites with high affinity for "hard" metal ions such as Al.sup.3+ and Fe.sup.3+, and there are sites with equally high affinities for such "soft" ions as Hg.sup.2+, Ag.sup.+ and Au.sup.3+. Selectivity is gained by judicious manipulation of solution parameters. For instance, chromate/dichromate, which are bound negligibly at pH values around neutrality, can be bound completely at pH 2.0. See for example, D. Darnall et al., "Recovery of Heavy Metal Ions by Immobilized Algae", Trace Metal Removal from Aqueous Solution, Special Publication No. 61, The Royal Society of Chemistry, Burlington House London, p. 3 (1986).
The difference in algal binding capacities for different metal ions is characterized using the concept of hard, intermediate and soft metal ions. If A is a metal ion and :B is a ligand, the stability of the complex, A:B, depends on the soft or hard character of A and :B. Metal ions are classified as hard or soft according to the order of their formation constant with the ligands F.sup.-, Cl.sup.-, Br.sup.-, and I.sup.--. For example, a metal ion is classified as hard if the stability of its complexes increases according to EQU I.sup.- &lt;Br.sup.- &lt;Cl.sup.- &lt;F.sup.-
and classified as soft if the stability of its complexes increases according to EQU F.sup.- &lt;Cl.sup.- &lt;Br.sup.- &lt;I.sup.-.
Some metal ions show intermediate behavior, and are classified as intermediate ions. For a more detailed discussion of hard, intermediate and soft metal ions, see for example, R.G. Pearson, "Hard and Soft Acids and Bases," J. Amer. Chem. Soc. 85, pp. 3533-3539 (1963) and J.E. Huheey, Inorganic Chemistry: Principles of Structure and Reactivity, Harper and Row, New York (1972).
As used herein "hard metal ions" refers to the group of metal ions consisting of Cs.sup.+, Rb.sup.+, Ba.sup.2+, Ra.sup.2+, B.sup.3+, Ge.sup.4+, Se.sup.4+, Se.sup.6+, V.sup.5+, Mn.sup.2+, Mn.sup.7+, Mo.sup.6+, W.sup.6+, Re.sup.7+, Y.sup.3+, Sn.sup.4+, Al.sup.3+, Sc.sup.3+, Ga.sup.3+, In.sup.3+, La.sup.3+, Cr.sup.6+, Cr.sup.3+, Co.sup.3+, Fe.sup.3+, As.sup.3+, As.sup.5+, Ir.sup.3+, Ce.sup.4+, Gd.sup.3+, Lu.sup.3+, Th.sup.4+, U.sup.4+, Pu.sup.4+, Ti.sup.4+, Zr.sup.4+, U.sup.+6, Hf.sup.4+, and the other metal ions from the lanthanide series. "Intermediate metal ions" refers to the group of metal ions consisting of Fe.sup.2+, Co.sup.2+ , Ni.sup.2+, Cu.sup.2+, Zn.sup.2+, Sn.sup.2+, Pb.sup.2+, Sb.sup.3+ and Bi.sup.2+. "Soft metal ions" refers to the group of metal ions consisting of Pd.sup.2+, Pt.sup.2+, Rh.sup.3+, Ir.sup.2+, Ru.sup.3+, Os.sup.2+, Pt.sup.+4 Cu.sup.+, Ag.sup.+, Au.sup.3+, Au.sup.+, Cd.sup.2+, Hg.sup.+ and Hg.sup.2+.
The binding site diversity of algae gives the algae a broad applicability not found in conventional ion-exchange resins A second major advantage with algae is that, in contrast to many conventional resins, the algal cells have relatively little affinity for Ca.sup.2+ and Mg.sup.2+. Thus, in hard-water treatment applications, the algae are less prone to saturation by these nontoxic ions. See for example, D. Darnall et al., "Recovery of Heavy Metals By Immobilized Algae," at pg. 3.
Metal ions can also be divided into three classes based upon the pH dependence of metal ion-binding to algae. The first class, Class I, is comprised of metal ions which are tightly bound at pH&gt;5 and which can be stripped (or are not bound) at pH&lt;2. Many ions fall into this class: Al.sup.+3, Cu.sup.+2, Pb.sup.+2, Cr.sup.+3, Cd.sup.+2, Ni.sup.+2, Co.sup.30 2, Zn.sup.+2, Fe.sup.+3, Be.sup.+2 and UO.sub.2.sup.+2. The second class, Class II, is comprised of metallic anions which display the opposite behavior of Class I metal ions, i.e., they are strongly bound at pH&lt;2 and weakly bound or not bound at all at pH values near 5. Ions in Class II include PtCl.sub.4.sup.-2, CrO.sub.4.sup.-2, MoO.sub.4.sup.-2 and SeO.sub.4.sup.-2. The third class of metal ions includes those metal ions for which there is no discernable pH dependence for binding between pH 6 and pH 1 and includes Ag.sup.+, Hg.sup.+2 and AuCl.sub.4.sup.-. These three ions are among the most strongly bound of all metal ions. FIGS. 1A, 1B and 1C illustrate data for the three classes of metal ions. For a more detailed discussion of the pH dependence of metal ion binding to algae, see for example, D. Darnall et al., "Recovery of Heavy Metals By Immobilized Algae," pp. 4-23.
The data in FIGS. 1A, 1B and 1C were collected by incubating Chlorella cells (5 mg/ml) in 0.1 mM solutions of the metal ions in 0.05 M acetate buffer. The buffer was added to maintain accurate pHs between pH 4 and 6. Because acetate is a good ligand for many of the metal ions, increased binding is observed in the absence of the buffer. Furthermore, complete binding (greater than 99%) of metal ions in all three classes is obtained when solutions are passed through columns containing immobilized algae rather than by simply incubating the algae in metal-containing solutions.
The pH dependence of metal ion binding to algae should make possible repeated cycles consisting of binding metal ions to algae and subsequently stripping the bound metal ions from the algae much like recycling an ion-exchange resin. Unfortunately, when algae cells are packed into a column and waters containing metal ions are passed through the column, the algae clump together and significant flow cannot be achieved even with high pressures. This problem is alleviated by immobilizing the algae. As used herein, "immobilization of algae," "immobilizing the algae," "immobilized algae" and similar terms refer to algae constrained within a composition. The immobilized algae can then be packed into columns through which high flow can be achieved.
Several different methods are known for immobilizing algae. Nakajima et al. (see Nakajima et al., "Recovery of Uranium By Immobilized Microorganisms," Eur. J. Appl. Microbiol. Biotechol., 16, pp. 88-91 (1982)) investigated a method for immobilizing Chlorella and Streptomyces in polyacrylamide, toluene diisocyanate, glutaraldehyde, agar, cellulose acetate and alginate Cells immobilized with polyacrylamide, toluene diisocyanate, glutaraldehyde and agar had the highest adsorption abilities. The cells immobilized with polyacrylamide had the best mechanical properties, such as rigidity. Nakajima et al. selected the polyacrylamide method as the most appropriate method for immobilization of algae. The polyacrylamide-immobilized alga functioned satisfactorily for removing uranium from both fresh and sea water at ambient temperatures. The adsorption of uranium by the immobilized cells was endothermic. However, the polyacrylamide composition is not very durable At alga concentrations above 20% (on a dry weight basis), the material is extremely prone to fracture.
An alternative to polyacrylamide-embedded material is an alga-silica composition. (See Darnall et al., "Recovery of Heavy Metals By Immobilized Algae," pp. 18-21). While the alga-silica composition is extremely hard ("rocklike") and resists fragmentation, the composition is porous so that all potential metal ion binding sites are capable of being occupied. The algal content of the polymer can be made as high as 90% (on a dry weight basis).
The alga-silica material functions as a chromatographic matrix and is highly durable. The material has been subjected to as many as 40-50 cycles of binding and elution (using Au.sup.3+) without observing any decrease in binding capacity. Furthermore, storage at room temperature for as long as two years at pHs ranging from less than about 1 to about 3 has no deleterious effect on the binding capacity of either gold or copper ions. The latter observation suggests that the silica-immobilized algal cells are not readily susceptible to microbial degradation. As the storage pH increases above about 3, the storage medium must be sterile or a growth inhibitor must be added to prevent the growth of unwanted microorganisms.
Other methods for algae immobilization have been investigated for application in areas other than extraction of metals from aqueous solutions. These methods for algae immobilization include use of alginate, polyurethane foam blocks, glass beads, agar, polyurethane, carrageenan and combinations of these substances. For a more detailed description of these alternative methods of immobilization see for example, P.K. Robinson, et al. "Immobilized Algae; A Review," Process Biochemistry, Vol. 8, p. 115 (1986).
The pH dependence of metal ion binding to algae, as indicated above, can be used to recover metal ions extracted from the aqueous solution by the algal compositions. For example, a column packed with beads of the alga-silica composition has been used to extract individual metal ions from an aqueous solution containing a mixture of metal ions To separate the individual ions from a mixture of Zn.sup.2+, Cu.sup.2+, Hg.sup.2+, and AuCl.sub.4.sup.-, the metal ions were loaded on a short column of immobilized C. vulgaris at pH 6.0. After washing the column thoroughly at the same pH, Zn.sup.2+ and Cu.sup.2+ were sequentially eluted by means of a pH gradient. The Hg.sup.2+ was then collected by elution with 0.5 M 2-mercaptoethanol at pH 2 and gold was collected by elution with the same reagent at pH 5.0. (See Darnall et al., "Recovery of Heavy Metal Ions by Immobilized Algae.")
The silica-algae composition is satisfactory for the recovery of metal ions in an aqueous solution when the pH values are less than 7. However, in some applications metal ions or complexes of metal ions must be recovered from an alkaline solution or a high temperature solution. Also, an alkaline eluent may be required to effectively strip metal ions bound to immobilized algae.
The previously described silica-algae compositions are not suitable for recovery of metal ions in aqueous solutions having temperatures greater than about 50.degree.-100.degree. C. because the compositions deteriorate as the temperature increases. In an aqueous solution having a temperature of about 160.degree. C., the silica-algae composition disintegrates within about one hour. This is because both the silica and the algae in the silica-algae composition hydrolyze at higher temperatures. Moreover, even at ambient temperatures, the silica-algae composition eventually hydrolyzes at pH values above 7 making the material unsuitable for recovery of metal ions in solutions having alkaline pHs. Hence, the silica-algae composition is not suitable for (i) the recovery of metal ions from an aqueous solution at high temperatures; (ii) the recovery of metal ions from an aqueous solution at alkaline pHs; or (iii) the extraction of metal ions bound to the composition using an eluent having a pH greater than 7.
The unique characteristics of algae and other materials, such as bacteria, fungi and plant materials, for binding metal ions suggest that a means for immobilization of these materials suitable for use with alkaline solutions and high temperatures might provide a mechanism for concentrating and removing metal ions from all aqueous solutions rather than certain aqueous solutions, as previously described.
Another prior art method for recovering gold cyanide complexes uses activated carbon. R. Bansal et al., Active Carbon, Marcel Dekker, Inc., New York, Chap. 1 (1988). Usually, charcoal is made by first heating biologically derived materials at temperatures of 400-800.degree. C. in a continuous stream of an inert gas. The charcoal formed by the initial heating does not have high adsorption capacity compared to activated carbon because charcoal has a poorly developed pore structure and low surface area. The charcoal is heated at temperatures of 850.degree. C. to 1100.degree. C. in the absence of air to produce activated carbon.
An alternative method of producing activated carbon uses a chemical activation process whereby the biological material is mixed with activating agents such as phosphoric acid, zinc chloride, sulfuric acid, or other chemicals. In this process the material is then heated at temperatures between 400.degree. and 600.degree. C. in the absence of air and then the material is cooled and washed to remove the activating agent After washing, the material is further heated at temperatures of 400.degree. to 800.degree. C.
The binding of gold cyanide to activated carbon is exothermic with the gold binding capacity of the activated carbon decreasing by nearly an order of magnitude as the temperature increases from 22.degree. C. to 79.degree. C.
The exothermic adsorption reaction behavior of activated carbon is the basis for extraction of bound gold from activated carbon. Specifically, the activated carbon is used to adsorb gold cyanide at ambient temperatures and then the temperature of the activated carbon is elevated in the presence of an eluting reagent, such as cyanide, to strip the bound gold from the activated carbon Accordingly, activated carbon is ineffective for removing gold from an aqueous solution at temperatures above 80.degree.-100.degree. C.
In addition to heating biological material to form active carbon, some investigators have heated compositions containing microorganisms to produce water insoluble particles In one published experiment, 100 grams of yeast cells were mixed with 0.5 grams of a water soluble polymer. (Japanese Laid-Open Patent Publication No. 49-121355, entitled "Method of Treating Waste Water By Water-Insoluble Microorganism Cells" of Kobayashi et al., dated Nov. 20, 1974.) The mixture was air dried to form a powder of about one millimeter thick particles. Using this procedure, particles containing immobilized yeast cells were formed using several different water soluble polymers The investigators reported that the particles were heated at temperatures ranging from 120.degree. C. for one hour to 160.degree. C. for two hours to form the water insoluble particles. The temperature and the duration of the heating were selected based upon the water soluble polymer used to form the composition. To demonstrate the insoluble nature of the particles, the particles were placed in 60.degree. C. water for three hours. The investigators stated that most of the particles maintained their integrity after the three hour exposure This result suggests that these compositions are likely to disintegrate upon exposure to high temperature solutions for several days or weeks.
Kobayashi et al. also mixed methyl vinyl ether:maleic anhydride copolymer with 10 grams of a green alga. Most of the moisture in the mixture was removed and then the mixture was passed through a small orifice to form a thread shape. The thread was cut into minute particles. After the particles were dried, Kobayashi et al. heated the dried particles at 120.degree. C. for one hour to form water insoluble particles.
In yet another experiment by Kobayashi et al. to form water insoluble particles, 100 grams of bread yeast were mixed with particles of methyl vinyl ether:maleic anhydride copolymer in water and then heated at 120.degree. C. for one hour. This process was repeated to produce insoluble particles of about 0.5 mm diameter. Ten grams of the insoluble particles were placed in a glass tube and an aqueous solution containing 10 ppm of Cd derived from CdCl.sub.2 was passed through the column 500 milliliters at a time. The flow rate through the column was about 700 milliliters per hour. The effluent from the column was analyzed and Kobayashi reported that cadmium was not detected until passage of the thirteenth 500 milliliter sample through the column. The binding capacity of the particles was 0.65% (gm Cd/gm material).
In another immobilization process, as described in the May 9, 1975 Japanese Laid-Open Patent Publication No. 50-51988 entitled "Method For Manufacturing Adsorbents for Water Treatment" of Ito et al., bacteria were reduced to a powder by drying in a vacuum at a temperature of 30.degree. C. The dried bacteria were sprayed with water drops. The resultant particles were rotated on a plate to form one to two millimeter diameter particles. Ito et al. reported that these particles were sealed in a nitrogen gas atmosphere and heated at temperatures ranging from 215.degree. C. to 250.degree. C. for one hour to form water insoluble particles. If the bacterial particles were heated at less than 215.degree. C., Ito et al. reported that the particles did not maintain their shape in water. Therefore, the bacterial particles that were heated at less than 215.degree. C. were unsuitable for use in a column to extract metal ions from an aqueous solution.
Ito et al. investigated the metal ion binding capacity of insoluble particles, formed as described above, as a function of the heating temperature used in the formation process. Ito et al. reported that insoluble particles formed by heating at 215.degree. C. adsorbed Hg to the extent of 3.8% of the dry weight of the particles while insoluble particles formed by heating at 250.degree. C. adsorbed very little Hg. In another experiment, Ito et al. reported that insoluble particles formed by heating at 270.degree. C. adsorbed cadmium from an aqueous solution, but the adsorption was less than the adsorption of particles formed by heating at 215.degree. C. Thus, according to Ito et al., as the heating temperature used to form water insoluble particles increased, the metal ion adsorption of the particles decreased.
Several techniques have been investigated to increase the adsorption capability of immobilized microorganisms which had been heated in a nitrogen atmosphere, as described above. See for example, the Dec. 18, 1974, Japanese Laid-Open Patent Application entitled "Adsorbent for Water Treatment Using Microorganisms" of Ito et al., and the May 8, 1975 Japanese Laid-Open Patent Application No. 50-51481 entitled "Adsorbent for Water Treatment" of Ito et al. In one experiment, bacteria particles were coagulated in a nitrogen atmosphere at 212.degree. C. for one hour. The resulting material was broken into 1 mm diameter particles and 5.5 grams of the 1 mm particles were placed in a glass tube. Ito et al. then flushed the tube with 10 mM hydrochloric acid at a flow of 300 ml/h until the pH of the effluent from the tube was about 2. The column containing the particles was then washed with 20 milliliters of water. Ten mM potassium hydroxide was flowed at 75 ml/hour through the column until the pH of the effluent became about 10. The column was again flushed with water.
The volume of the particles in the column expanded to 13 ml after the acid treatment and to 23 ml after the alkali treatment. Thus, although the particles are insoluble, the acid-alkali treatment resulted in significant swelling of the particles. This swelling is undesirable for applications in which the compositions are used in a closed chamber because the swelling is likely to result in either rupture of the chamber or blockage of flow through the chamber.
Ito et al. repeated the acid treatment process on the same insoluble particles several times. After each acid-alkali cycle, an aqueous solution containing 100 ppm of Cd derived from CdCl.sub.2 was passed at 500 ml/h through the column until the concentration of the cadmium in the effluent was 10 ppm. The Cd adsorbed by the unprocessed composition was 0.91% of the dry weight of the particles. The Cd adsorption increased to 2.6% of the dry weight of the particles after six acid-alkali process cycles. Thus, the acid-alkali treatment increased not only the volume of the particles but also the Cd adsorption by about a factor of 3.
In yet another attempt to increase the adsorption of insoluble particles formed by heating in a nitrogen atmosphere at 215.degree. C., Ito et al. soaked the insoluble particles overnight at room temperature in an aqueous potassium hydroxide solution having a pH of 12. The particles which precipitated from the potassium hydroxide aqueous solution were placed in a glass column and washed with water until the pH of the effluent from the column became about 7.4. In a control experiment, Ito et al. soaked another group of particles overnight in water. The Cd adsorption capability of particles was about 0.97% of the dry weight of the material for the water soaked particles the potassium hydroxide treated particles. Similar adsorption increases were seen for the adsorption of Pb and basic methyl violet after treating the particles with potassium hydroxide.
The increased adsorption capability obtained by the alkali treatment was attributed by Ito et al. to the enhanced porosity of the particles. The solubility of structural cell wall material is higher in an alkaline liquid than in a neutral or acidic liquid. Thus, according to Ito et al., the soaking of the insoluble particles in the alkaline solution resulted in solubilization, and consequently removal, of organic and inorganic substances with low molecular weights, i.e., structural cell wall material, from the composition which in turn enhanced the porosity of the composition.
While the above compositions were heated at temperatures as high as 270.degree. C. in a nitrogen atmosphere to form water insoluble compositions, as the heating temperature used to form the compositions increased, the metal ion adsorption capability of the compositions decreased. These results suggest that particles containing immobilized metal ion-binding microorganisms which are heated at temperatures greater than 270.degree. C. would probably not adsorb metal ions from an aqueous solution effectively. These results are supported by the process used to form active carbon. As described above, charcoal which is formed by heating biological material at 400.degree.-800.degree. C. in an inert atmosphere, has relatively poor adsorption capability. To obtain a material with effective adsorption capability, the charcoal is further processed at 850.degree. C.-1100.degree. C. to form active carbon. However, as previously discussed, active carbon is not useful for adsorption of metal ions from an aqueous solution having a temperature of 80.degree. C. or greater. Accordingly, based upon the above experience with biological materials, heating of immobilized microorganisms at temperatures in the range of about 250.degree. C. to 800.degree. C. would not be expected to form compositions suitable for extraction of metal ions from an aqueous solution at either ambient or elevated temperatures.